There are many electrical apparatus whose main function lies in opening or closing a pair of electrical contacts in response to physical changes in the environment such as heat, electric current value, pressure or the like. In the case of this kind of electrical apparatus, whenever an anticipated physical change takes place in the apparatus, the electrical contacts are either opened or closed accordingly. Such electrical apparatus includes, for example, battery protectors, motor protectors, thermostats, pressure switches, circuit breakers, relays, etc. A battery protector or a motor protector is installed between the battery or motor and the target electrical product such as a portable computer, camera, communications equipment, automobile control equipment, or the like. In the case where an over-current has been produced in a battery (or in a motor), first and second contacts of the protector are separated from one another, thereby protecting the electrical product involved. In the case of a thermostat, the contacts are opened or closed in response to a selected change in the ambient temperature, thereby shutting off the power source and thus functioning in such a way as to maintain the ambient temperature constant.
A problem common to all of these electrical apparatus having such contacts is erosion or wear of the contacts due to arc discharges which are produced upon opening or closing of the contacts. When contacts that have been in engagement are separated from each other in the state where electric current has been impressed, an arc discharge is produced, with a result that a part of the contact material is eroded. By repeated opening and closing actions of the contacts, the contact material is reduced in a gradual manner and this limits the life of the apparatus.
Manufacturers of this kind of electrical apparatus, determine the expected number of cycles during the useful life of the apparatus due to wearing of the contacts by electric arc discharge and discourage use of the apparatus in excess of this number. In actuality, however, there are cases where these electrical apparatus are used in excess of the expected number. In such cases, it has been impossible to tell with conventional electrical apparatus if the final failure mode will end in contact welding or in a non-conductive state. In the former case, therefore, the electric apparatus does not function as desired and there is a danger of damaging the electrical product for which protection is intended.